The detection of defects on the surface of semiconductor wafers due to imperfect production or the post-production adhesion process has received considerable attention in the art. Generally, wafers fall into two main categories, “unstructured” (or “unpatterned”) and “patterned”. A patterned wafer has circuit patterns (“dies”) imprinted on it, while an unstructured (unpatterned) wafer is still bare, i.e. with no circuits imprinted on it as yet.
Generally speaking, numerous systems and methods have been developed to cope with the problem of defect detection and in particular, for the non-destructive inspection of silicon wafers. A prior art system known as the “Excite System” of Applied Materials includes a light beam source and an optical system that projects the beam onto the test object, as well as means for detecting the reflected and/or scattered light. There is an additional assembly for moving the test object in a coordinated translational and rotary movement, so that the light spot projected thereon scans the whole surface along a spiral path. The detected scattered light is analyzed in order to determine the sought defects.
The development of processes enabling the manufacture of wafer surfaces with ever-finer structures, urged the development of inspection systems for the detection of ever more minute defects such as particle contamination, polishing scratches, variations in the thickness of coatings, roughness, crystal defects on and below the surface, etc. Insofar as unstructured wafers are concerned, they are subjected to a thorough searching examination for detecting said defects.
In the chip manufacturing process, it is common to monitor each stage in order to recognize problems as early as possible and thus avoid undue waste. When unstructured wafers are compared between two process stages, the types and amount of defects at some stage can be determined. The inspected surface may be rough and metallized, and may therefore produce a great deal of scattered light, or, it may be a film-coated surface with a small amount of defects and produce scattered light. Thus, the inspecting instrument should preferably have a wide dynamic range of detection to permit defect and particle detection of a wide variety of surfaces.
Laser scanners are particularly suitable for that purpose. Note that presently available laser scanners differ in the type of scanning they use, their optical configuration, and the manner in which the results are processed. For applications that require a high throughput and nearly 100% inspection of the whole wafer surface, two processes are mainly used. In the first, disclosed e.g. in U.S. Pat. No. 4,314,763 to Steigmeier & Knop, the illuminating beam and the collecting optics are stationary, and the test object is scanned spirally by means of a coordinated translational and rotary movement of the object itself. In the second process disclosed, e.g. in U.S. Pat. No. 4,378,159 to Galbraith, a rotating or vibrating mirror moves the illuminating beam in one direction linearly back and forth across the wafer, while the wafer is simultaneously translated perpendicular thereto. In general, the first method is simpler and with homogenous accuracy, while the second is faster.
Bearing all that in mind, attention is drawn to U.S. Pat. No. 6,271,916 to Marxer et al. Briefly speaking, the Marxer patent discloses an assembly for non-destructive surface inspections. The system according to the '916 patent will now be briefly described with reference to FIGS. 1A-B. Thus, the apparatus according to the Marxer patent includes a light beam that is directed by a beam deflector 113 and 131 towards the wafer's surface 135, preferably normal thereto. The wafer is moved by a rotation motor 145 and a translation motor 149 according to the technique disclosed in the '159 patent. A circumferential ellipsoidal mirrored surface 127 is placed around the wafer, with its axis coinciding with the surface normal, to collect scattered light from defects at the wafer surface at collection angles away from the surface normal. In some applications, a lens arrangement with its axis coinciding with the surface normal is also used to collect the light scattered by the surface and by any defects on it. The light scattered by the mirror and lenses may be directed to the same or different detectors. Preferably, light scattered by the surface within a first range of collection angles from the axis is detected by a first detector 121, and light scattered by the surface within a second range of collection angles from the axis is detected by a second detector 125 (shown in FIG. 1B only). The two ranges of collection angles are different, with one detector optimized to detect scattering from large defects (mainly large particles) and the other detector optimized to detect light from small defects (particles). The content of the Marxer patent is incorporated herein by reference.
The detectors according to the Marxer patent, detect practically only light scattered from defects, whereas reflected light (reflected from a well-polished surface) is out-guided in order not to interfere with the scattered light received by the detectors. This method of measuring diffused light from defects only is called “dark field”.
The apparatus according to the Marxer patent offers a solution applicable, if at all, to the detection of defects on unstructured wafers. However, the specified apparatus of the Marxer patent is not applicable to the detection of defects on patterned wafers, because in the case of patterned wafers, the detectors do not only receive light scattered from defects, but also light scattered from the patterns. Considering that the intensity of the latter is much higher than that of the former, it would be very difficult and in fact practically infeasible to determine whether the received light originates from a defect or from a fault-free pattern.
Die to die defect analysis is based upon a comparison (usually a on a pixel to pixel bases or even a sub-pixel to sub-pixel bases) of pixels originating from light scattered from the same spot on two distinct dies. Die to die comparison require hat substantially the same illumination and collection conditions apply during the generation of the pixels. The Marxer patent does not enable die to die defect analysis as the wafer is rotated during the illumination of the wafer, and both the illumination and collection paths constantly change as result from the wafers rotation. The problem is especially acute when the wafers are patterned and when using dark field detectors to detect defects, as the dark field images are very dependent upon the direction of light scattered from the rotating pattern. Accordingly, there is a need in the art to provide an apparatus that performs defect detection of patterned wafers.
There is another need in the art to provide an apparatus that performs defect detection of both patterned and unpatterned wafers.
There is yet a further need to allow a compact optical inspection apparatus that enables die to die defect analysis.